1. Field of the Invention
The present invention relates to a metal gasket assembly comprised primarily of metal components such as stainless steel plates.
2. Description of the Related Art
A conventional metal gasket assembly generally comprises one or more metal plates having openings and beads formed around the openings. Such a metal gasket assembly is typically used in an internal combustion engine as a seal between a cylinder block and a cylinder head.
To improve the sealing effect of a metal gasket assembly, there has been proposed to attach a shim to a substrate metal sheet at a region surrounding a cylinder opening so that the fastening pressure applied to the metal gasket assembly is locally increased in that region.
However, the conventional metal gasket assembly interposed between the cylinder block and the cylinder head requires a strong fastening by bolts for effective sealing, and thus it may cause a deformation in the cylinder head. Particularly, in a portion around a combustion chamber, the deformation in the cylinder head tends to increase the size of a gap formed between the cylinder block and the cylinder head. This gap allows an escape of combustion gases from the chamber, which leads to a contamination of the metal gasket assembly and hence, to a gradual deterioration of the seal between the cylinder block and the cylinder head.
Moreover, since the size of the gap around the combustion chamber changes corresponding to the change of combustion pressure in the internal combustion engine, beads provided on the metal gasket assembly are subjected to an alternate load of compression and restoration, thereby giving rise to fatigue of the beads on the metal gasket assembly. The fatigued beads with reduced elasticity weaken the fastening function of bolts, with the result that the sealing effect of the metal gasket assembly is decreased.
Furthermore, in the case of a metal gasket assembly employing a shim ring, the thickness of the shim ring is significantly influential on the sealing performance of the metal gasket assembly. However, it is difficult to adjust the shim thickness to 0.05 mm or less, because of difficulties in producing, with a sufficient accuracy, a plate of such thickness for the use as a shim.
It is therefore a primary object of the present invention to provide an improved metal gasket assembly which is free from the disadvantages inherent in the conventional metal gasket assembly.
To attain the above-mentioned object, a metal gasket assembly according to the present invention comprises a substrate metal sheet with at least one opening, and a peripheral member fitted into the opening along the periphery thereof, which peripheral member is made of a metal plate having a thickness larger than that of the substrate metal sheet.
With the above-mentioned arrangement in accordance with the present invention, since the thickness of the peripheral member is larger than that of the substrate metal sheet, the fastening by bolts is particularly effective in the peripheral member, thereby improving the sealing performance at that portion of the metal gasket assembly.
Moreover, when a metal gasket assembly of the present invention is used in an internal combustion engine for a seal between a cylinder block and a cylinder head, an opening of the metal gasket assembly for a combustion chamber is formed by the circumferentially inner periphery of a peripheral member. Accordingly, a gap between the cylinder block and the cylinder head caused by the fastening of bolts can be made smaller at the region around the combustion chamber due to the peripheral member which has a larger thickness than the substrate metal sheet of the metal gasket assembly. As a result, it can obviate such an escape of combustion gases from the combustion chamber that causes a contamination of the metal gasket assembly, thereby preventing decrease of sealing effect.
The smaller gap reduces the alternate load, which is caused by the combustion pressure within the combustion chamber and works on beads of the metal gasket assembly, thereby suppressing fatigue of the beads. This also contributes to prevention of decrease in the sealing effect.
Since an increase in thickness at the periphery of openings is produced by using a peripheral member made of a separate metal plate having a larger thickness than a substrate metal sheet, instead of superposing a shim ring on the peripheral portion of the substrate metal sheet, an increase of 0.05 mm or less can be easily and accurately achieved by the present invention.
The present invention also provides a method for producing a metal gasket assembly, which comprises the steps of:
perforating a substrate metal sheet so that the substrate metal sheet has at least one aperture larger than a desired opening;
fitting a metal plate member having a thickness greater than that of the substrate metal sheet into the aperture along the periphery thereof and welding them together; and
perforating the welded metal plate member to form the desired opening thereon.
Since it is after the completion of the welding that the desired opening is provided on the metal plate member to form it into a peripheral member, the width of the peripheral member is never affected by the welding in the present invention. Accordingly, the metal gasket assembly of the present invention can be produced with an excellent dimensional accuracy even when the peripheral member is 1-2 mm in width, because it is free from deformations due to welding.